Common rail systems for heavy diesel engines require large accumulator volumes for hydraulic reasons. Hence result—also for manufacturing reasons—large sealing diameters for sealing the high-pressure-loaded components. In terms of thread load, those large sealing diameters are disadvantageous in threaded connections. On the one hand, high prestressing forces have to be used and, on the other hand, the dynamic load on the thread is increased by the pulsating internal pressure load.
At present, a relief of the thread can practically and effectively only be achieved by increasing the base radius of the thread—in order to reduce the notch effect—and by increasing the diameter of the thread—which entails an increase in the force-transmitting surface. A positive influence on the load-carrying capacity of the screw connection can, moreover, be achieved by an improved material quality, an increased pitch, in particular with highly hardened and tempered screw connections, heat treatment techniques, thread manufacturing (final tempering—final rolling), and via the lubrication state. Geometrically, the use of tension nuts and threads with flank angle differences as well as high screw-in depths may be helpful for the fatigue strength.
All of the usable measures mentioned are, however, limited in their effects, in particular where high-prestressing forces are required.